Pauline Galea

The Easter Sunday 2011 Earthquake Swarm Offshore Malta: Analysis on Felt Reports

In the last forty years, Malta has only experienced a few occasional tremors from local or regional earthquakes, with only some being reported briefly in the local newspapers. These reports gave limited qualitative and quantitative information about the shaking experience felt across the islands. The Seismic Monitoring and Research Unit at the University of Malta has put in place a ‘Did you feel an earthquake?’ online questionnaire in order to start gathering information from locally felt earthquake related shaking. On Easter Sunday 24th of April 2011 windows rattled, doors shook open, and furniture shifted in many homes across Malta when a series of earthquakes occurred 38 km off the eastern coast. In total, the SMRU located 15 earthquakes with magnitudes ranging from M L 1.8 to 4.1 over a period of 4 days. A total of 489 felt reports were submitted through the online questionnaire. The compilation of the data is a first of its kind for the Maltese islands. Here we present a summary of the reports following the main shock. A maximum intensity value IV on the European Macroseismic Scale was assigned. No structural damage was reported. The data reflects the demographics as well as the different types of buildings found across the archipelago.

Performance evaluation of Wied Dalam (WDD) seismic station in Malta

The continual operation of a permanent seismograph, now exceeding a couple of decades in some cases, naturally involves changes of hardware and soft- ware over time. Nonetheless, the long-term, consistent performance of the seismic station, and the good quality of its data, is very important for national seismic stud- ies investigating the local seismicity, and also important for the international seismological community research- ing regional tectonics and deep Earth structures. Here we investigate the data availability and quality of the currently only seismic station on Malta (WDD) since its installation in 1995, and establish spectral patterns in the seismic data that may be inuenced by diurnal vari- ations, seasonal weather changes, and/or site-specic settings. The results are important for the future de- ployment of permanent seismic stations on the Maltese islands, and for the analysis of local seismic hazard and ground motion studies.

Earthquake and People: The Maltese Experience of the 1908 Messina Earthquake

On December 28, 1908 at 5:20 a.m. local time, a devastating earthquake (Mw = 7.2) struck Southern Italy along the Messina Strait (Fig. 1). This event caused severe ground shaking throughout the region and triggered a local tsunami. As result the cities of Messina along Sicily’s coast and Reggio di Calabria were completely destroyed (Baratta, 1910) causing more than 120,000 fatalities and with many left without shelter. The effects of the earthquake were felt within a 300-kilometer radius. Rescuers searched through the rubble for weeks, and people were still being pulled out alive days later, but thousands remained buried there. The 1908 earthquake had a significant impact on buildings and people and local communities which were displaced. The Maltese experience of the Messina 1908 earthquake relied on communication which reached Malta after the event. The assessment of the Maltese experience of the Messina Earthquake has so far been carried out with reference to published newspaper reports and other brief accounts including Herbert Ganado’s Rajt Malta Tinbidel. Alfons Maria Galea a Maltese author and filanthropist published a book in Maltese on the earthquake and its devastating effects in the popular educational series il-Kotba tal-Mogħdija taz-Żmien just a few weeks after the event. The book is a vivid account of the destruction caused by the earthquake, the suffering of the survivors and the reaction of the population in reviving the city. The document presents first-hand accounts of the events in sufficient detail to give a clear picture of the severity of the event, extents of the damage and impact on the population. It is mostly based on accounts received by Galea from persons in institutions including religious orders in Sicily who he knew. Newspaper reports in Malta and other countries together with Galea’s book present clear first-hand accounts of this event and provide information on the building deficiencies and damage, limitations of communication infrastructure during that period, limits to timely emergency response to support the population and emergency action at the beginning of the 20th century.

Earthquake ground-motion simulations for the Maltese Archipelago

The main goal of this paper is to provide earthquake ground motion simulations for particular earthquake scenarios, in terms of ground motion parameters for the Maltese islands. We used a stochastic approach to simulate high-frequency strong-ground motions, using an extended-source model code. This code was developed for earthquake simulations using stochastic finite-fault modelling and a dynamic corner frequency approach. The extended-source model code is a reliable and practical method to simulate ground motion records of moderate and large earthquakes especially in regions where structural damage is expected, but sparse ground motion recordings are available. In this paper, we show that in the Maltese archipelago, the ground motion from the repeat occurrence of historically recorded earthquakes, or from other potential sources, coupled with existing geological conditions and building typologies has the potential to cause significant structural damage in the area.

Integrated GPR and passive seismic investigations at cultural heritage sites: Case studies in Malta

In this paper we report results from an integrated measurement campaign performed on the island of Malta. Both GPR and seismic noise data were gathered in two sites close to the sea, where two watchtowers built by the Order of St. John are located. The two investigations were performed on the top of the cliff inGolden Bay, close to the Ghajn Tuffieha Tower, and at the Madliena Tower in Pembroke. The main goal of the survey was to investigate the possible presence of natural or man-made elements that might affect the preservation of these monuments in the future. The investigations were therefore both on the two historical buildings and on a portion of soil nearby to also check the geological stability of the cliff since the towers are located right on its edge. The measurements were carried out with an innovative stepped frequency GPR system and with passive seismic instrumentation. In the following the main achievements will be presented.

Evaluation of seismic site response in the Maltese archipelago

The investigation of local ground conditions is an important part of seismic hazard assessment (Fäh et al., ). It is now well–established that earthquake ground shaking is not only a function of the earthquake magnitude and epicentral distance, but also of the site conditions, including soft layers in the sub–soil stratification and topographical features. Local geology can greatly alter the seismic waves from earthquakes by amplifying their amplitude, changing the frequency content and increasing the shaking duration during an earthquake (Kramer, ). In fact, several unconsolidated soft sites have suffered significantly greater damage than rock sites. One case was the  Michoacán earthquake which showed low peak ground acceleration near the epicenter, yet caused severe damage in Mexico City, which is found more than  km away and is characterized by soft shallow sediments (Campillo et al., ). The main parameters responsible for such effects are the shear– wave velocity (VS) structure and thickness of the sedimentary cover, the impedance contrast between the soft sediments and the underlying bedrock as well as the geometry of their interface (Parolai et al.,). Knowledge of the VS structure and/or the resonance frequency of soft soil layers is of utmost importance for the prevention or mitigation of earthquake disasters (Arai and Tokimatsu, ). Such information should contribute to earthquake–hazard mitiga-

Overview of the seismic hazard in the Sicily channel archipelagos

A joint Italo–Maltese research project (Costituzione di un Sistema Integrato di Protezione Civile Transfrontaliero Italo–Maltese, SIMIT) was financially supported by the European community. One of the aims of SIMIT was to improve the geological and geophysical information in Lampedusa and in Malta and ultimately to mitigate natural hazards. Although this region lies on the Sicily Channel Rift Zone, a seismically active domain of Central Mediterranean, the knowledge about seismotectonic and seismic hazard is not satisfactory. At present, seismic hazard assessment (SHA) for Italy (MPS Working Group, ), Tunisia (Ksentini and Romdhane, ) and more generally for whole European areas (Giardini et al., ) do exist, whereas no specific SHA for the Sicily channel archipelagos are available. The Sicily Channel appears to be a region of moderate seismic activity, with the seismicity mainly located in the surrounding areas (Fig. ). For the Malta archipelago a first catalogue, listing historical and felt earthquakes, was made by Galea (), whereas the Database Macrosismico Italiano (DBMI; Locati et al., ) does not list any data as regards earthquakes felt in Lampedusa. For this reason, in the present study, a theoretical seismic history was derived (Fig. ) for Lampedusa and Malta, using the European–Mediterranean Earthquake Catalogue (EMEC) (Grünthal and Wahlström, ) and the attenuation relationship for macroseismic intensity data by Pasolini et al. (). The two study areas do not appear to have been affected by strong earthquakes occurring in the Sicily channel, but they were somehow struck by major earthquakes occurring in the surrounding area. Although the present description seems to exclude large shaking effects, SHA for the study region is

Prediction of High-Frequency Ground Motion Parameters Based on Weak Motion Data

This research was carried out using computational facilities procured through the European Regional Development Fund, Project ERDF-080 ‘A supercomputing laboratory for the University of Malta (http://www.um.edu.mt/research/scienceeng/erdf_080).